1. Field of the Invention
This invention relates to a vehicle transmission, and more particularly to a vehicle transmission that includes a wet clutch installed on a crankshaft.
2. Description of the Related Art
Referring to FIG. 1, a conventional vehicle transmission 1 is adapted to transmit power from an engine of a vehicle, such as an all-terrain vehicle, a motorcycle, etc., to a rear wheel (not shown) of the vehicle. The conventional vehicle transmission 1 includes a crankshaft box unit 12 driven by the engine, a clutch unit 13 disposed in proximity to the crankshaft box unit 12, and a transmission box unit 14 disposed in proximity to the clutch unit 13.
The crankshaft box unit 12 includes a crankshaft box 125, a crankshaft 121 disposed in the crankshaft box 125 and rotatable by the engine, a driven shaft 122 having two ends connected respectively and pivotally to the crankshaft 121 and a transmission box 140 of the transmission box unit 14, a first bearing 123 disposed between the transmission box 140 and the driven shaft 122, and a self-lubricating bearing 124 sleeved on the driven shaft 122.
The clutch unit 13 includes a seal cover 135 cooperating with the crankshaft box 125 of the crankshaft box unit 12 to define a clutch chamber 130, a wet clutch 131 sleeved on the crankshaft 121 and rotatable with the crankshaft 121, a sleeve body 134 sleeved fixedly on the driven shaft 122, an outer shield 132 connected fixedly to the sleeve body 134 by a plurality of rivets 136 (only two are shown) and disposed around the wet clutch 131, and a second bearing 133 disposed between the sleeve body 134 and the crankshaft 121. The driven shaft 122 extends through the seal cover 135. When the crankshaft 121 rotates, the wet clutch 131 comes into frictional contact with the outer shield 132 so as to allow for co-rotation of the driven shaft 122 and the crankshaft 121.
The transmission box unit 14 includes a driving pulley unit 141 sleeved on the driven shaft 122, a coupling shaft (not shown) connected fixedly to the rear wheel, a driven pulley unit (not shown) sleeved on the coupling shaft, and a V-belt 143 trained on the driving pulley unit 141 and the driven pulley unit so as to transfer rotation of the driving pulley unit 141 to the driven pulley unit. When the driven shaft 122 rotates at a speed smaller than a threshold speed, the pitch diameter of the driving pulley unit 141 is smaller than that of the driven pulley unit. In this state, the rotation speed of the rear wheel is smaller than that of the driven shaft 122. When the driven shaft 122 rotates at a speed greater than the threshold speed, the pitch diameter of the driving pulley unit 141 is greater than that of the driven pulley unit. In this state, the rotation speed of the rear wheel is greater than that of the driven shaft 122.
When the engine is in an idle speed condition, although the crankshaft 121 and the wet clutch 131 rotate, the wet clutch 131 is spaced apart from the outer shield 132. As such, power cannot be transmitted from crankshaft 121 to the driven shaft 122 and, thus, the rear wheel. When a throttle is operated to increase the rotation speed of the crankshaft 121 and the wet clutch 131, the wet clutch 131 engages the outer shield 132 so as to allow for transmission of power from the crankshaft 121 to the driven shaft 122. When the engine is in an acceleration condition, the pitch diameter of the driving pulley unit 141 increases, while the pitch diameter of the driven pulley unit is reduced, thereby resulting in an increase in the rotation speed of the rear wheel. When the engine is in a deceleration condition, the pitch diameter of the driving pulley unit 141 reduces, while the pitch diameter of the driven pulley unit is increased, thereby resulting in a decrease in the rotation speed of the rear wheel.
Since the crankshaft 121 and the driven shaft 122 are coupled to each other at a position between the wet clutch 131 and the outer shield 132, many elements need to cooperate with the crankshaft 121, the driven shaft 122, the wet clutch 131, and the outer shield 132 in a highly precise manner. This results in difficulties with respect to assembly of these elements.